In most laser applications it is desirable to be able to vary and control output power of the laser. This is true for lasers that deliver output radiation at a fundamental wavelength of a gain-medium of the laser and also true for lasers in which fundamental radiation is converted to second, third, fourth or even higher harmonic output radiation in one or more optically nonlinear crystals.
In the latter case it has been common practice to vary the harmonic output by varying the power of the fundamental radiation that is converted to harmonic radiation. The fundamental power is typically varied by varying energy (pump-power) delivered to the gain-medium for energizing the gain-medium. A problem with this method is that varying the pump-power usually varies thermal conditions of the gain-medium. In solid-state lasers a thermal condition that varies is thermal lensing. Variation of thermal lensing can cause transient effects including variations in beam quality. It is particularly the case for Q-switched and other pulsed, solid-state lasers.
Several methods have been proposed to minimize thermal lensing effects when varying output power in a solid state laser. Two such methods are described in U.S. Pat. No. 6,414,980, and in U.S. Pat. No. 6,683,893, each thereof assigned to the assignee of the present invention. In each case the methods are dependent on the mode of operation of a Q-switch controlling the pulse-repetition rate and the average output power of the laser. U.S. Pat. No. 6,115,402, also assigned to the assignee of the present invention, describes a method of compensating thermal lensing changes resulting from varying pump-light power by using a movable resonator mirror. While these methods are effective, they are not perfect. Generally, the bigger the range of power variation required, the less effective the method, at least over some transient period. There is a need for an alternative method for varying and controlling output-power in harmonic-generating lasers.